Cancer of the breast is a major cause of death among the American female population. Effective treatment of this disease is most readily accomplished following early detection of malignant tumors. Major efforts are presently underway to provide mass screening of the population for symptoms of breast tumors. Such screening efforts will require sophisticated, automated equipment to reliably accomplish the detection process.
The x-ray absorption density resolution of present photographic x-ray methods is insufficient to provide reliable early detection of malignant tumors. Research has indicated that the probability of metastasis increases sharply for breast tumors over 1 cm in size. Tumors of this size rarely produce sufficient contrast in a mammogram to be detectable. To produce detectable contrast in photographic mammograms, 2-3 cm dimensions are required. Calcium deposits used for inferential detection of tumors in conventional mammography also appear to be associated with tumors of large size. For these reasons, photographic mammography has been relatively ineffective in the detection of this condition.
Most mammographic apparatus in use today in clinics and hospitals require breast compression techniques that are uncomfortable at best and in many cases painful to the patient. In addition, x-rays constitute ionizing radiation, which adds a further risk factor into the use of mammographic techniques as most universally employed.
Ultrasound has also been suggested as in U.S. Pat. No. 4,075,883, which requires that the breast be immersed in a fluid-filled scanning chamber. U.S. Pat. No. 3,973,126 also requires that the breast be immersed in a fluid-filled chamber for an x-ray scanning technique.
In recent times, the use of light and more specifically laser light to noninvasively peer inside the body to reveal the interior structure has been investigated. This technique is called optical tomography. Rapid progress over the past decade has brought optical tomography to the brink of clinical usefulness. Optical tomography has the benefits compared to mammography of no breast compression and no ionizing radiation.
The patient lies prone on a scanning apparatus with one breast pendent in a scanning chamber. A laser beam impinges upon the breast; light is scattered throughout the breast and detected by an array of optical detectors. The scanning apparatus acquires data from one or several slices through the breast, parallel to the chest wall. The detection mechanism then moves some small distance away from the chest wall and the data for more slices are acquired. This process continues until the entire breast has been imaged.
One complication, and the subject of the present invention, is that the shape of the breast varies from patient to patient and particularly changes dramatically as the scanning proceeds from the chest wall toward the nipple. It is an absolute requirement of a 3rd generation optical CT scanner that the center of rotation of the scanning mechanism lay within the breast. It is desirable that the center of rotation of the scanning mechanism lies near the center of the breast. But breasts do not hang vertically in the prone position. Cooper's Droop occurs when Cooper's ligaments stretch over time, causing the breasts to sag when upright. In the prone position, breasts hang somewhat towards the feet as the scanning apparatus progresses from the chest towards the nipple (see FIG. 3A). A breast that is well centered at the chest wall through the axis of rotation of the scanning mechanism usually will be off-center near the nipple.